1. Field of the Invention
This invention relates to a novel method for making silica optical devices and more particularly such devices by means of sol-gel processing and creating therein thermally derived index gradients and to silica and silica gel optical devices as produced by the practice of the method.
2. Description of the Prior Art
Present methods of producing optical waveguides involve the use of chemical composition gradients induced either by chemical diffusion or ion exchange at the surface of a glass. Thermal fusion of dissimilar composition of glass can also be used. The chemical composition gradients result in a variation of index of refraction which can be used to guide optical signals. Any of these methods has inherent difficulties in controlling the shape of the index of refraction gradient. In addition, the presently used chemical methods for making wave guides have intrinsic limitations in the dimensions of the regions of differing index of refraction. A further disadvantage of chemical methods of inducing index gradients is that they cannot be used to produce internal, three-dimensional optical waveguides, i.e., present methods are restricted to production of planar or channel waveguides. Even further, the common use of alkali ion exchange to make waveguides in soda-lime-silica glass leads to an intrinsic mismatch of index of refraction of the waveguide to pure, vitreous silica fibers used in optical transmission systems.
Thus, there is great need for pure silica optical waveguides and optical components that are compatible with the index of refraction of optical transmission lines. Also, chemically induced index gradients suffer the possibility of compositional changes with time due to atmospheric attack or damage by imposed electromagnetic radiation. Waveguides carrying high intensity light can induce changes at the interface of the chemical composition gradients which result in a progressive increase in scattering and losses.
Furthermore, because of the refractory nature of pure SiO.sub.2 it is very difficult to produce precise index gradients by chemical compositional diffusion. Consequently, it is exceedingly difficult to produce optical couplers, multiplexers, and other components in pure SiO.sub.2 that can match the index of refraction of fiber optics transmission lines.